1. Field of the Invention
The present disclosure relates to a nonvolatile magnetic memory device, and more particularly, to a nonvolatile magnetic memory device including a magnetic tunnel junction (MTJ).
2. Description of the Related Art
Examples of a nonvolatile memory device include a phase-change random access memory (PRAM), a magnetoresistive RAM (MRAM), a ferroelectric RAM (FRAM), and a resistive RAM (ReRAM). Among these nonvolatile memory devices, the MRAM is a solid-state magnetic memory device using a giant magnetoresistance effect or a tunneling magnetoresistance effect based on a spin-dependent conductivity of a nano magnetic material. The MRAM has recently attracted attention because of its higher speed and greater durability, even after repeated use, than other nonvolatile memory devices.
A magnetic tunnel junction (MTJ) is a storage node structure of an MRAM. The MTJ structure, in which one insulating layer is inserted between two magnetic thin films, may be coupled to a conventional complementary metal oxide semiconductor (CMOS) architecture.
A magnetic field writing (MFW) method and a current-induced magnetization switching (CIMS) method may be used to write data in cells of an MRAM having the MTJ structure. Using these methods, the cells of the MRAM may store information in an in-plane magnetization (IPM) manner or a magnetization-perpendicular-to-a-plane (MPP) manner.
IPM has a disadvantage in that it is difficult to scale down a size of the MRAM, due to sensitivity to an anisotropic shape and an edge effect, which increase as the area of the MTJ decreases. In contrast, MPP has an advantage over IPM in terms of scaling down the MRAM, since MPP reduces switching abnormalities related to the edge effect.
The MTJ using MPP may exhibit magnetoresistance switching behavior due to bulk anisotropy or interface anisotropy of a magnetic thin film. In general, bulk anisotropy requires a high level of writing current for field switching or spin transfer torque. Moreover, since a high degree of crystallinity of the magnetic thin film is necessary to achieve bulk anisotropy, high manufacturing temperatures equal to or higher than 500° C. are required.
Accordingly, it is advantageous to use MPP using the interface anisotropy for low-current switching and fabrication at a lower temperature equal to or lower than 300° C. However, due to a low magnetization crystalline anisotropy (Ku), MPP that uses the interface anisotropy has poor thermal stability, which adversely affects data retention capabilities of the MRAM.
CoFeB has been suggested as a new material suitable for an MPP-MTJ method (see “A perpendicular-anisotropy CoFeB—MgO magnetic tunnel junction”, S. Ikeda, etc., Nature Materials, Volume 9, Pages 721 to 724 (2010)). However, since the addition of boron reduces spin polarization, loss of a magnetoresistance in CoFeB may be greater than in other materials such as Co, CoFe, or NiFe having a crystalline structure.